Hi Domi,
Throwing a quick glimps at the OsmoTRX wiki page I think the cheapest hardware that should work well is the LimeSDR - coming in at around 300 USD.
Looking around myself, I came to the same general conclusion - but there are several different members of the Lime family. It looks like LimeSDR-Mini is the cheapest of all of them, but does it have some problem compared to the original LimeSDR-USB? Does it have a worse clock that isn't good enough to run a GSM BTS? And what about LimeNET-Micro? It appears to be priced the same as the original LimeSDR-USB, but has a built-in RPi microcomputer that the software would run on, and its GPSDO seems like a better clock option than all of the predecessors - but I could be missing something...
M~